Re: os cache vs. db cache
- From: Robyn <robyn.sands@xxxxxxxxx>
- To: "Christo Kutrovsky" <kutrovsky.oracle@xxxxxxxxx>
- Date: Wed, 19 Dec 2007 16:09:54 -0500
Hello Christo,
You may have joined late but your timing is impeccable. I've recently
changed roles and the system this question pertained to is no longer my
headache. I had been thinking about testing a minimized OS cache
configuration, but due to several of the answers received, I realized that I
had a mixed bag of queries to contend with:
1. Some queries had been tuned for direct path and parallelization
2. Some queries were extremely untuned and possibly benefiting from the os
cache
3. Some queries would have been better served by increasing Oracle cache(s)
and reducing the os cache size
There were some plans to rework most of the DWH as part of an upgrade
project, however, those plans have been set aside and I am on to new
projects, working on linux sans SANs. I'm setting up a new test bed this
week, and should have an opportunity to test several variations of the
configuration. So I will keep your comments in my notes ... they will be
very useful in the new year.
thank you ... Robyn
On Dec 19, 2007 12:42 PM, Christo Kutrovsky <kutrovsky.oracle@xxxxxxxxx>
wrote:
> Joining a bit late into this converation.
>
> Here's my opinion on OS vs Oracle cache vs PGA target. Strictly with
> linux in mind.
>
> I would *always* go for the Oracle cache. The reason for this is not
> so much the more efficient hot/cold management.
> - The Oracle cache is more efficient. Using the Oracle cache reduces
> your CPU usage (your *licensed* cpu) as well as is more concurent then
> the OS cache. The Oracle cache can be used on the spot, there is no
> extra work associated, just use it.
> - I can monitor the Oracle cache, i can see which objects are
> consuming it, how much dirty data and etc, all on a very granual level
> - I get to have *real* disk responce time statistics. As opposed to a
> blended rate due to OS cache
> - I have more control of what goes into cache and when. I have control
> how fast to write out dirty data to disk.
>
> Same goes for the PGA. I would always give more PGA then OS cache. Of
> course, there's the special case where each "work area" can be at most
> 4gb (even on 64 bit) but that can be worked arround using parallelism,
> where each parallel process can get it's 4gb.
> - I get to see which areas are taking up space, which areas spilled to
> disk, which queries are cached which are not.
> - Perhaps I can the the ones that are causing disk activity - you
> loose all this info with a blended OS cache.
> - It's more efficient, no extra CPU work required
> - It avoids double buffering - you will always have double buffering
> if you rely on OS cache.
> - The last thing you want on a CPU intensive task is more CPU to be used.
>
> Now mind you not, the default settings for PGA and sort/hash related
> writes are very unfriendly for the datawarehouse, but tunning these
> you can achieve some exceptional performance. Up to 2x on a going to
> TEMP simply by tunning the IO patterns. SAN cache or not.
>
> I just cant see the case where the OS cache would be more advantegous.
>
> Christo Kutrovsky
> DBA Team Lead
> The Pythian Group
>
>
> On 7/10/07, Robyn <robyn.sands@xxxxxxxxx> wrote:
> > Jonathan,
> >
> > Thank you very much for your reply. One of the systems in question is a
> > warehouse and the issues you mention are very relevant. This database
> has
> > been around since '97 or so, and the range of queries and reports is
> huge,
> > so of course, the range of plans and methods for data access are varied
> as
> > well. I have been working on tuning some of the key queries, and I am
> seeing
> > more queries use direct access reads, but it is not yet the primarily
> access
> > method. However, I'm thinking the best approach for this database is to
> > continue tuning for more parallelization and more direct reads, and
> perhaps
> > increasing the os cache and decreasing the database cache will be
> effective
> > after we've made more progress on that front. It's certainly something
> I
> > hope to see although I've got quite a bit of work to do first.
> >
> > However, I'm just getting starting on moving warehouse data into
> partitioned
> > tables and indexes (just got licensing for this option recently) so
> again we
> > don't currently have a lot partition index reads now but they should be
> > increasing soon. If partitioned indexes continued to use the buffer
> cache,
> > I suppose this could change the mix as well. Either way, I've got a lot
> of
> > work to do first ...
> >
> > Our temporary space is already on a cached files system although I
> expect
> > that was by accident, so I'm glad to hear that we may be benefiting from
> > this configuration. I'll check into the PGA aggregate target
> suggestion;
> > hashing and sorting is a problem area for several of the queries I'm
> > concerned about this week and I'd noticed a decline in PGA cache ratio
> from
> > where it was about 3 months ago. We have a year end closing coming up
> and
> > our revenue data will be extremely active in the next few weeks.
> >
> > Thank you again ...
> >
> > Robyn
> >
> >
> > On 7/10/07, Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > Robyn,
> > >
> > > Since you're running a data warehouse, you may want to consider
> > > the effects of parallel tablescans and parallel index fast full scans,
> > > which do direct path reads and bypass the buffer cache.
> > >
> > > Of course, there are parallel execution paths that still USE
> > > the buffer cache (parallel nested loop joins with indexed access,
> > > access through partitioned indexes etc.) but if a very large
> > > percentage of the work you do is bypassing the Oracle cache,
> > > you may want to switch memory to the file system cache instead
> > > to try and keep some important objects "close to" the action.
> > >
> > > Alternatively, you might think about putting the temporary tablespace
> > > onto a filesystem that has caching enabled so that blocks written to
> the
> > > temp tablespace are in the cache (and not on disc) when you start to
> > > re-read them.
> > >
> > > Yet another option is to keep the caching low - depending on a SAN
> > > cache (for exanple) to help you with the TEMP space - and make as
> > > much memory as you can available for PGA activity so that you minimise
> > > the number of cases where sorts or hash joins spill to disc.
> > >
> > > It's not a clear cut choice, unfortunately - often it's a question of
> > observing
> > > the current activity and making the best guess about how moving memory
> > > around will benefit the critical processes.
> > >
> > >
> > >
> > >
> > > Regards
> > >
> > > Jonathan Lewis
> > > http://jonathanlewis.wordpress.com
> > >
> > > Author: Cost Based Oracle: Fundamentals
> > > http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html
> > >
> > > The Co-operative Oracle Users' FAQ
> > > http://www.jlcomp.demon.co.uk/faq/ind_faq.html
> > >
> > >
> > > ----- Original Message -----
> > > > Date: Mon, 9 Jul 2007 14:03:42 -0400
> > > > From: Robyn <robyn.sands@xxxxxxxxx >
> > > > Subject: os cache vs. db cache
> > > >
> > > > Hello everyone,
> > > > Over the years, I've read differing opinions on balancing the os
> cache
> > and
> > > > the database cache. Seems like I remember there were some advocates
> for
> > > > minimizing the os cache and maximizing the database cache to allow
> > Oracle to
> > > > better determine which blocks should truly be cached and avoid
> necessary
> > > > blocks being cached because they were in proximity to hot data. I
> also
> > have
> > > > heard opposite opinions.
> > > >
> > > > I'm specifically focused on performance for a data warehouse. Has
> > anyone
> > > > tested performance with a minimized OS cache and a maximized
> database
> > > > cache? If so, what were the results? Does anyone have a really
> good
> > paper
> > > > or book on the topic? I'm trying to collect some information before
> I
> > start
> > > > discussions with the platforms guys.
> > > >
> > > > tia ... Robyn
> > > >
> > >
> > > --
> > > //www.freelists.org/webpage/oracle-l
> > >
> > >
> > >
> >
> >
>
>
> --
> Christo Kutrovsky
> Senior Database/System Administrator
> The Pythian Group - www.pythian.com
> I blog at http://www.pythian.com/blogs/
>
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